* make pg_hba authoption be a set of 0 or more name=value pairs

* make LDAP use this instead of the hacky previous method to specify
  the DN to bind as
* make all auth options behave the same when they are not compiled
  into the server
* rename "ident maps" to "user name maps", and support them for all
  auth methods that provide an external username

This makes a backwards incompatible change in the format of pg_hba.conf
for the ident, PAM and LDAP authentication methods.